Archipelago » qemu

@example

@c man begin SYNOPSIS

usage: qemu-img command [command options]

@c man end

@end example

@c man begin DESCRIPTION

qemu-img allows you to create, convert and modify images offline. It can handle

all image formats supported by QEMU.

@b{Warning:} Never use qemu-img to modify images in use by a running virtual

machine or any other process; this may destroy the image. Also, be aware that

querying an image that is being modified by another process may encounter

inconsistent state.

@c man end

@c man begin OPTIONS

The following commands are supported:

@include qemu-img-cmds.texi

Command parameters:

@table @var

@item filename

 is a disk image filename

@item fmt

is the disk image format. It is guessed automatically in most cases. See below

for a description of the supported disk formats.

@item --backing-chain

will enumerate information about backing files in a disk image chain. Refer

below for further description.

@item size

is the disk image size in bytes. Optional suffixes @code{k} or @code{K}

(kilobyte, 1024) @code{M} (megabyte, 1024k) and @code{G} (gigabyte, 1024M)

and T (terabyte, 1024G) are supported.  @code{b} is ignored.

@item output_filename

is the destination disk image filename

@item output_fmt

 is the destination format

@item options

is a comma separated list of format specific options in a

name=value format. Use @code{-o ?} for an overview of the options supported

by the used format or see the format descriptions below for details.

@item -c

indicates that target image must be compressed (qcow format only)

@item -h

with or without a command shows help and lists the supported formats

@item -p

display progress bar (convert and rebase commands only)

@item -q

Quiet mode - do not print any output (except errors). There's no progress bar

in case both @var{-q} and @var{-p} options are used.

@item -S @var{size}

indicates the consecutive number of bytes that must contain only zeros

for qemu-img to create a sparse image during conversion. This value is rounded

down to the nearest 512 bytes. You may use the common size suffixes like

@code{k} for kilobytes.

@item -t @var{cache}

specifies the cache mode that should be used with the (destination) file. See

the documentation of the emulator's @code{-drive cache=...} option for allowed

values.

@end table

Parameters to snapshot subcommand:

@table @option

@item snapshot

is the name of the snapshot to create, apply or delete

@item -a

applies a snapshot (revert disk to saved state)

@item -c

creates a snapshot

@item -d

deletes a snapshot

@item -l

lists all snapshots in the given image

@end table

Parameters to compare subcommand:

@table @option

@item -f

First image format

@item -F

Second image format

@item -s

Strict mode - fail on on different image size or sector allocation

@end table

Command description:

@table @option

@item check [-f @var{fmt}] [--output=@var{ofmt}] [-r [leaks | all]] @var{filename}

Perform a consistency check on the disk image @var{filename}. The command can

output in the format @var{ofmt} which is either @code{human} or @code{json}.

If @code{-r} is specified, qemu-img tries to repair any inconsistencies found

during the check. @code{-r leaks} repairs only cluster leaks, whereas

@code{-r all} fixes all kinds of errors, with a higher risk of choosing the

wrong fix or hiding corruption that has already occurred.

Only the formats @code{qcow2}, @code{qed} and @code{vdi} support

consistency checks.

@item create [-f @var{fmt}] [-o @var{options}] @var{filename} [@var{size}]

Create the new disk image @var{filename} of size @var{size} and format

@var{fmt}. Depending on the file format, you can add one or more @var{options}

that enable additional features of this format.

If the option @var{backing_file} is specified, then the image will record

only the differences from @var{backing_file}. No size needs to be specified in

this case. @var{backing_file} will never be modified unless you use the

@code{commit} monitor command (or qemu-img commit).

The size can also be specified using the @var{size} option with @code{-o},

it doesn't need to be specified separately in this case.

@item commit [-f @var{fmt}] [-t @var{cache}] @var{filename}

Commit the changes recorded in @var{filename} in its base image.

@item compare [-f @var{fmt}] [-F @var{fmt}] [-p] [-s] [-q] @var{filename1} @var{filename2}

Check if two images have the same content. You can compare images with

different format or settings.

The format is probed unless you specify it by @var{-f} (used for

@var{filename1}) and/or @var{-F} (used for @var{filename2}) option.

By default, images with different size are considered identical if the larger

image contains only unallocated and/or zeroed sectors in the area after the end

of the other image. In addition, if any sector is not allocated in one image

and contains only zero bytes in the second one, it is evaluated as equal. You

can use Strict mode by specifying the @var{-s} option. When compare runs in

Strict mode, it fails in case image size differs or a sector is allocated in

one image and is not allocated in the second one.

By default, compare prints out a result message. This message displays

information that both images are same or the position of the first different

byte. In addition, result message can report different image size in case

Strict mode is used.

Compare exits with @code{0} in case the images are equal and with @code{1}

in case the images differ. Other exit codes mean an error occurred during

execution and standard error output should contain an error message.

The following table sumarizes all exit codes of the compare subcommand:

@table @option

@item 0

Images are identical

@item 1

Images differ

@item 2

Error on opening an image

@item 3

Error on checking a sector allocation

@item 4

Error on reading data

@end table

@item convert [-c] [-p] [-f @var{fmt}] [-t @var{cache}] [-O @var{output_fmt}] [-o @var{options}] [-s @var{snapshot_name}] [-S @var{sparse_size}] @var{filename} [@var{filename2} [...]] @var{output_filename}

Convert the disk image @var{filename} or a snapshot @var{snapshot_name} to disk image @var{output_filename}

using format @var{output_fmt}. It can be optionally compressed (@code{-c}

option) or use any format specific options like encryption (@code{-o} option).

Only the formats @code{qcow} and @code{qcow2} support compression. The

compression is read-only. It means that if a compressed sector is

rewritten, then it is rewritten as uncompressed data.

Image conversion is also useful to get smaller image when using a

growable format such as @code{qcow} or @code{cow}: the empty sectors

are detected and suppressed from the destination image.

You can use the @var{backing_file} option to force the output image to be

created as a copy on write image of the specified base image; the

@var{backing_file} should have the same content as the input's base image,

however the path, image format, etc may differ.

@item info [-f @var{fmt}] [--output=@var{ofmt}] [--backing-chain] @var{filename}

Give information about the disk image @var{filename}. Use it in

particular to know the size reserved on disk which can be different

from the displayed size. If VM snapshots are stored in the disk image,

they are displayed too. The command can output in the format @var{ofmt}

which is either @code{human} or @code{json}.

If a disk image has a backing file chain, information about each disk image in

the chain can be recursively enumerated by using the option @code{--backing-chain}.

For instance, if you have an image chain like:

@example

base.qcow2 <- snap1.qcow2 <- snap2.qcow2

@end example

To enumerate information about each disk image in the above chain, starting from top to base, do:

@example

qemu-img info --backing-chain snap2.qcow2

@end example

@item snapshot [-l | -a @var{snapshot} | -c @var{snapshot} | -d @var{snapshot} ] @var{filename}

List, apply, create or delete snapshots in image @var{filename}.

@item rebase [-f @var{fmt}] [-t @var{cache}] [-p] [-u] -b @var{backing_file} [-F @var{backing_fmt}] @var{filename}

Changes the backing file of an image. Only the formats @code{qcow2} and

@code{qed} support changing the backing file.

The backing file is changed to @var{backing_file} and (if the image format of

@var{filename} supports this) the backing file format is changed to

@var{backing_fmt}. If @var{backing_file} is specified as ``'' (the empty

string), then the image is rebased onto no backing file (i.e. it will exist

independently of any backing file).

There are two different modes in which @code{rebase} can operate:

@table @option

@item Safe mode

This is the default mode and performs a real rebase operation. The new backing

file may differ from the old one and qemu-img rebase will take care of keeping

the guest-visible content of @var{filename} unchanged.

In order to achieve this, any clusters that differ between @var{backing_file}

and the old backing file of @var{filename} are merged into @var{filename}

before actually changing the backing file.

Note that the safe mode is an expensive operation, comparable to converting

an image. It only works if the old backing file still exists.

@item Unsafe mode

qemu-img uses the unsafe mode if @code{-u} is specified. In this mode, only the

backing file name and format of @var{filename} is changed without any checks

on the file contents. The user must take care of specifying the correct new

backing file, or the guest-visible content of the image will be corrupted.

This mode is useful for renaming or moving the backing file to somewhere else.

It can be used without an accessible old backing file, i.e. you can use it to

fix an image whose backing file has already been moved/renamed.

@end table

You can use @code{rebase} to perform a ``diff'' operation on two

disk images.  This can be useful when you have copied or cloned

a guest, and you want to get back to a thin image on top of a

template or base image.

Say that @code{base.img} has been cloned as @code{modified.img} by

copying it, and that the @code{modified.img} guest has run so there

are now some changes compared to @code{base.img}.  To construct a thin

image called @code{diff.qcow2} that contains just the differences, do:

@example

qemu-img create -f qcow2 -b modified.img diff.qcow2

qemu-img rebase -b base.img diff.qcow2

@end example

At this point, @code{modified.img} can be discarded, since

@code{base.img + diff.qcow2} contains the same information.

@item resize @var{filename} [+ | -]@var{size}

Change the disk image as if it had been created with @var{size}.

Before using this command to shrink a disk image, you MUST use file system and

partitioning tools inside the VM to reduce allocated file systems and partition

sizes accordingly.  Failure to do so will result in data loss!

After using this command to grow a disk image, you must use file system and

partitioning tools inside the VM to actually begin using the new space on the

device.

@end table

@c man end

@ignore

@c man begin NOTES

Supported image file formats:

@table @option

@item raw

Raw disk image format (default). This format has the advantage of

being simple and easily exportable to all other emulators. If your

file system supports @emph{holes} (for example in ext2 or ext3 on

Linux or NTFS on Windows), then only the written sectors will reserve

space. Use @code{qemu-img info} to know the real size used by the

image or @code{ls -ls} on Unix/Linux.

@item qcow2

QEMU image format, the most versatile format. Use it to have smaller

images (useful if your filesystem does not supports holes, for example

on Windows), optional AES encryption, zlib based compression and

support of multiple VM snapshots.

Supported options:

@table @code

@item compat

Determines the qcow2 version to use. @code{compat=0.10} uses the traditional

image format that can be read by any QEMU since 0.10 (this is the default).

@code{compat=1.1} enables image format extensions that only QEMU 1.1 and

newer understand. Amongst others, this includes zero clusters, which allow

efficient copy-on-read for sparse images.

@item backing_file

File name of a base image (see @option{create} subcommand)

@item backing_fmt

Image format of the base image

@item encryption

If this option is set to @code{on}, the image is encrypted.

Encryption uses the AES format which is very secure (128 bit keys). Use

a long password (16 characters) to get maximum protection.

@item cluster_size

Changes the qcow2 cluster size (must be between 512 and 2M). Smaller cluster

sizes can improve the image file size whereas larger cluster sizes generally

provide better performance.

@item preallocation

Preallocation mode (allowed values: off, metadata). An image with preallocated

metadata is initially larger but can improve performance when the image needs

to grow.

@item lazy_refcounts

If this option is set to @code{on}, reference count updates are postponed with

the goal of avoiding metadata I/O and improving performance. This is

particularly interesting with @option{cache=writethrough} which doesn't batch

metadata updates. The tradeoff is that after a host crash, the reference count

tables must be rebuilt, i.e. on the next open an (automatic) @code{qemu-img

check -r all} is required, which may take some time.

This option can only be enabled if @code{compat=1.1} is specified.

@end table

@item Other

QEMU also supports various other image file formats for compatibility with

older QEMU versions or other hypervisors, including VMDK, VDI, VHD (vpc), qcow1

and QED. For a full list of supported formats see @code{qemu-img --help}.

For a more detailed description of these formats, see the QEMU Emulation User

Documentation.

The main purpose of the block drivers for these formats is image conversion.

For running VMs, it is recommended to convert the disk images to either raw or

qcow2 in order to achieve good performance.

@end table

@c man end

@setfilename qemu-img

@settitle QEMU disk image utility

@c man begin SEEALSO

The HTML documentation of QEMU for more precise information and Linux

user mode emulator invocation.

@c man end

@c man begin AUTHOR

Fabrice Bellard

@c man end

@end ignore